Blood Pressure Variability and Cardiovascular Outcome in Essential Hypertension:

There is mounting evidence that average ambulatory blood pressure (BP) is superior to clinic BP in predicting cardiovascular risk in essential hypertension. The impact of ambulatory BP variability on cardiovascular outcome has also been evaluated. The independent prognostic relevance of BP variability in initially untreated hypertensive subjects has not yet been clearly established. Verdecchia et al,¹ in a previous analysis, reported that BP variability was not an independent predictor of a composite pool of events. Sander et al² showed a univariate association between daytime systolic BP variability and future events, but unfortunately multivariate analysis was not available. We³ did not find an independent association between daytime BP variability and cardiovascular outcome in mild hypertension. Bjorklund et al⁴ reported an independent association between daytime systolic BP variability and cardiovascular morbidity in a population of untreated and treated elderly men. When untreated subjects were analyzed separately, no independent association was found.⁴ Finally, in untreated elderly hypertensive subjects Pringle et al⁵ showed that night-time systolic BP variability was independently associated with increased risk of stroke, but not of cardiac events.

In the present issue of the American Journal of Hypertension, Verdecchia et al⁶ further analyzed this problem. They evaluated the impact of BP variability (daytime or night-time systolic and diastolic) on cardiac and cerebrovascular events analyzed separately in 2649 initially untreated hypertensive subjects. During the follow up 167 new cardiac and 122 new cerebrovascular events occurred. In multivariate analysis, after adjustment for other covariates, a high night-time systolic BP variability was associated with higher risk of cardiac events (high vs low BP variability, relative risk 1.51, 95% confidence interval 1.06–2.16, P = 0.024). Night-time BP variability did not predict cerebrovascular events and daytime BP variability was not an independent predictor of risk.

These results add further knowledge about the prognostic relevance of BP variability. The study by Verdecchia et al⁶ has the strength of a high number of events. Moreover, another advantage is the frequency of BP recording every 15 min throughout 24 h. However, they used fixed-clock time intervals to define daytime and night-time. This approach eliminates transition periods in the morning and in the evening, but also excludes an important period of the day (from 6:00 AM to 10:00 AM) during which morning BP surge could be relevant. In addition, data on the reproducibility of BP variability are lacking. The different prognostic impact of daytime and night-time BP variability is difficult to explain. Daytime BP variability may be conditioned by factors such as morning BP surge, orthostatic BP changes, physical activity, and stress. Although night-time BP variability could be considered more representative of the intrinsic BP variability, it could be influenced by factors such as sleep apnea, quality of sleep, and the frequency of awakening. Thus, it cannot be totally excluded that differences between subjects regarding these characteristics could have influenced the results to some extent. Another feature deserving mention is the different impact of BP variability on cardiac and cerebrovascular events. In the present study night-time systolic BP variability was associated with cardiac but not cerebrovascular events. This result is intriguing considering that BP changes are better related to stroke than to myocardial infarction. It is unclear whether high BP variability exerts its detrimental effect in the long term by inducing organ damage or in the short term by damaging atherosclerotic lesions and causing acute events. In the latter hypothesis it remains difficult to explain the association between night-time BP variability and cardiac events that frequently occur during daytime.

In any case, this study adds further insight into the multifaceted association between BP variability and cardiovascular outcome in initially untreated hypertensive patients.

However, additional large studies taking into account confounders of daytime and night-time BP variability are needed to finally establish whether BP variability is independently associated with cardiovascular outcome in hypertension, whether daytime and night-time BP variability have a different relevance, and whether the adverse impact regards all events or specific events.